We have demonstrated striking differences in the metabolism of phosphatidylcholine from perinatal and adult rabbit lung. Specifically, using isotopic labeling techniques and a number of different radio-labeled phospholipid precursors, premature newborn, term newborn, 3 day old, and premature and in utero rabbits are unable to secrete labeled phosphatidylcholine or disaturated phosphatidylcholine into the alveolar wash (surfactant) fraction of lung until 3 hours after isotope administration. However, once secreted, the labeled phospholipids remain in the alveolar wash of the perinatal animals and are apparently turned over very slowly. In contrast, adult animals rapidly secrete labeled phosphatidylcholine, which is then turned over rapidly with half-life values for alveolar phosphatidylcholine of 14-35 hours, depending on the labeled precursor used. The labeling patterns of phospholipids in microsomal and lamellar body lung subcellular fractions during development are being used to further define the subcellular locations of differences between adult and perinatal animals. The kinetics of incorporation of various phospholipid precursors into phosphatidylcholine and disaturated phosphatidylcholine are being measured very soon (0.75 min to 2 hours) after precursor administration. Ultra-centrifugation techniques are being used to develop methods for the isolation of the "growing" forms of the surfactant secretory granule - the lamellar body, in an effort to study the "development" of the lamellar body as a subcellular organelle. These studies should help define surfactant metabolism in both the perinatal and adult animal.